1. Field of the Invention
The invention relates to a fuel cell system.
2. Description of Related Art
A fuel cell is a device that, by continuously replenishing fuel as well as discharging waste by-products of the fuel, directly converts chemical energy of fuel to electrical energy. In general, fuel cells are characterized by high power generating efficiency, reduced amounts of air pollutants, and low noise.
A related fuel cell structure may include a plurality of cells, or power generator units. Each cell may include sets of gas diffusion electrodes and corresponding separators that are attached to opposite sides of an electrolyte. Each of the separators may include a respective gas passage. Electrical power is generated in a cell when a fuel gas, such as hydrogen gas or the like, is supplied via the fuel gas passage to the fuel gas diffusion electrode (also called a “fuel electrode”) attached on one side of the cell and when an oxidizing gas, such as air containing oxygen, is supplied via the oxidizing gas passage to the oxygen gas diffusion electrode (also called an “oxygen electrode” or an “oxidizing electrode”) attached on the opposite side of the cell. The fuel is oxidized and a change of free energy that is induced thereby produces electrical energy.
A related fuel cell known as a polymer electrolyte fuel cell includes a membrane electrode assembly (MEA) in each cell. In particular, a polymer electrolyte membrane includes electrode catalyst layers that are disposed on opposite sides of the membrane, and separators that supply different gases to the surfaces of the catalyst layers on the opposite sides of the membrane. A first separator supplies hydrogen as the fuel gas to the fuel electrode disposed on a first side of the membrane, and a second separator supplies air as an oxidizing gas to the oxidizing electrode disposed on a second side of the membrane. In the related fuel cell, the electrode catalyst layers include a catalyst such as platinum on a carbon carrier.
The related polymer electrolyte membrane exhibits hydrogen ion conductivity when containing water, and thus, the fuel gas that is fed to the fuel cell is usually humidified in advance. Furthermore, since the reaction at the oxidizing electrode produces water, water is constantly contained in the cell.
Accordingly, after a long period of operation by the related fuel cell, various ionic impurities and inorganic and organic impurities contained in carbon materials, as well as seal materials, plastic materials and metallic materials used in the construction of the related fuel cell, are dissolved in the water contained in the cell. Furthermore, air that is supplied to the fuel cell from the outside inevitably contains air pollutants, such as small quantities of nitrogen oxides and sulfur oxides, and the fuel gas tends to gather metallic oxides during passage through a hydrogen purifier.
These various impurities mentioned above are collected on the electrolyte membrane and the catalyst reaction layers of oxygen electrode and fuel electrode, which reduces conductivity of the polymer electrolyte and diminishes activity of the catalyst reaction. As a result, the cell performance gradually deteriorates over time as the fuel cell is operated. Furthermore, in a related fuel cell in which the separators are formed with metal plates, the electrolyte membrane and catalyst reaction layers may be significantly damaged by metal ions dissolved from the metal separators.
An example of a related fuel cell system is disclosed in Japanese Laid-open Patent Application 2001-85037, which describes applying a high current density load to a fuel cell that has deteriorated in performance.